Chemically selective imaging provides a direct approach to unraveling cellular machinery and disease mechanisms. Our long term goal is to develop highly sensitive optical microscopy that allows selective imaging of single macromolecules or single molecular assemblies in live cells without fluorophore labeling, and to apply such tools for mapping intermediate metabolites in disease processes and drug pathways. The recently developed nonlinear optical microscopy based on coherent anti-Stokes Raman scattering (CARS) allows vibrational imaging with a high signal level. However, the nonresonant background limits the sensitivity and spectral selectivity. This proposal aims to push the detection sensitivity and molecular selectivity of CARS microscopy to a new level. We propose to develop three-color CARS microscopy in which three synchronized picosecond pulse trains are used to acquire on-resonance and off-resonance CARS image simultaneously. The vibrationally resonant CARS signal is extracted from the difference between the two images. We plan to apply three-color CARS microscopy to test the hypothesis of rafts in cellular membranes. We have been able to image specific lipids in model membranes using the CARS signal from the aliphatic CD stretch vibration of a lipid molecule with fully deuterated acyl chains. [unreadable] [unreadable]